Anion/hydroxide exchange membrane fuel cells (AEMFCs/HEMFCs) have received increasing attention due to their dominant advantages such as (a) more facile fuel oxidation and oxygen reduction in high pH media, (b) electro osmotic drag by OH− from cathode to anode, which not only reduces fuel crossover but also realizes anode drainage, and (c) complete elimination of the crippling bi/carbonate contamination problem of traditional liquid alkaline fuel cells (AFCs) whose electrolyte contains free metal cations. See, e.g., C. Lamy, E. M. Belgsir, J. M. Leger, Journal of Applied Electrochemistry 31, 799 (2001); Y. Wang, L. Li, L. Hu et al., Electrochemistry Communications 5 (8), 662 (2003); J. R. Varcoe and R. C. T. Slade, Fuel Cells 5 (2), 187 (2005).
A suitable anion/hydroxide exchange ionomer (i.e., a charged polymer) is crucial and of foremost importance to build three-phase boundaries in the electrodes. Unfortunately, unlike high performance acid Nafion ionomer for proton exchange membrane fuel cells (PEMFCs), high performance hydroxide exchange ionomer has not heretofore been available for AEMFCs/HEMFCs. This greatly limits AEMFCs/HEMFCs' performance and development.
Due to the lack of a solid ionomer, KOH or NaOH aqueous solution has been used in the electrodes, which limits advantages of AEMFCs/HEMFCs over traditional AFCs. See, e.g., K. Matsuoka, Y. Iriyama, T. Abe et al., Journal of Power Sources 150, 27 (2005); E. H. Yu and K. Scott, Journal of Power Sources 137 (2), 248 (2004); E. Agel, J. Bouet, and J. F. Fauvarque, Journal of Power Sources 101 (2), 267 (2001); L. Li and Y. X. Wang, Journal of Membrane Science 262 (1-2), 1 (2005); C. Coutanceau, L. Demarconnay, C. Lamy et al., Journal of Power Sources 156 (1), 14 (2006). Non ionic conductive PTFE has also been used as ionomer, which doesn't provide OH− transfer in the electrode at all. See, e.g., E. H. Yu and K. Scott, Journal of Applied Electrochemistry 35 (1), 91 (2005). Sometimes acid Nafion ionomer was used as ionomer, which restrains the OH− transfer in the electrode dramatically. See, e.g., H. Y. Hou, G. Q. Sun, R. H. He et al., Journal of Power Sources 182 (1), 95 (2008); A. Verma and S. Basu, Journal of Power Sources 174 (1), 180 (2007).
Recently, an insoluble cross linked di-amine quaternized polyvinyl benzyl electrochemical interface was prepared to enhance HEMFC performance. See, e.g., J. R. Varcoe, R. C. T. Slade, and E. Lam How Yee, Chemical Communications (13), 1428 (2006); J. R. Varcoe and R. C. T. Slade, Electrochemistry Communications 8 (5), 839 (2006). But this polymer is not a soluble ionomer, and thus it cannot be used to effectively build three-phase boundaries in electrodes, and as a result the HEMFC performance is still far limited. In addition, its ionic conductivity and stability is also limited due to its alkaline quaternary ammonium hydroxide group. Very recently, a soluble alkaline ionomer, A3-solution (Tokuyama) was reported; however, its chemical structure, preparation method, and characterized properties such as ionic conductivity, stability, and fuel cell performance, are unknown. See, e.g., H. Bunazawa and Y. Yamazaki, Journal of Power Sources 182 (1), 48 (2008).